The warmer feeling is palpable on the House floor during Obama's State of the Union address. The Gabrielle Giffords shooting weighs heavily on the mood.

Reporting from Washington — Matched in bipartisan pairings and eager to spotlight a softer side of Congress, lawmakers showed off their best behavior Tuesday as they bowed, sometimes awkwardly, to recent calls for a more civil political discourse.

Wearing black-and-white ribbons in honor of Rep. Gabrielle Giffords, the Arizona Democrat critically wounded in a Tucson shooting rampage Jan. 8, Republicans and Democrats mingled on the House floor, abandoned their usual partisan seating plan, shook hands and gave warm hugs, and, occasionally, joined together in standing ovations.

The warmer feeling was palpable on the floor, said Sen. Richard J. Durbin, a Democrat from Illinois who sat next to Republican Sen. Mark Steven Kirk (R-Ill.).

"It was a lot different. It was like kids in school that are grown up enough for the girls to sit with the boys," Durbin said. "There was much better conduct when they sit together."

The Giffords shooting weighed heavily on the mood in the chamber, which was far from the pep rallies of years past. A seat with the Arizona delegation was left empty in tribute.

"We are part of the American family," President Obama told the crowd.

And perhaps like a family, the slights, arguments and habits of the past were not easy to abandon. Democrats largely sat on the left and were quick to jump to their feet to applaud the president. Republicans were sometimes reluctant to stand — even as Obama called for programs they tend to endorse, such as tax reform. House Majority Leader Eric Cantor (R-Va.) and Minority Leader Nancy Pelosi (D-San Francisco), after flirting with the idea of sitting together, chose other seatmates.

Rep. Paul Broun (R-Ga.) tweeted some 40 times during the speech, including this rebuke: "Mr. President, you don't believe in the Constitution. You believe in socialism."

Still, online criticism was a less confrontational approach than that taken in Obama's other recent addresses to Congress. In a 2009 joint session, Rep. Joe Wilson (R-S.C.) yelled, "You lie!" as the president spoke about provisions in his healthcare bill. During last year's State of the Union address, Supreme Court Justice Samuel A. Alito Jr. shook his head in disagreement and mouthed the words "Not true" as Obama criticized the court's ruling on corporate political expenditures.

Alito did not attend Tuesday's speech — he had accepted an invitation to teach law in Hawaii. But in keeping with the theme of the night, there was what could be called a bipartisan majority of the Supreme Court in attendance. Six of the nine justices showed up, including Chief Justice John G. Roberts Jr. and Justice Anthony M. Kennedy, both nominated by Republicans. Neither Antonin Scalia nor Clarence Thomas attended, but they rarely do. They were absent last year as well.

The intentionally mixed seating was the first in recent memory for a State of the Union speech. The divided-seating scheme — Republicans on the right and Democrats on the left — was apparently well established by the time the speech first aired on evening television in 1965, according to the Congressional Research Service.

Sen. Mark Udall (D-Colo.) led the call to break from convention, and it was quickly adopted as a way to show that lawmakers were mindful of the subdued and cooperative political climate in the wake of the shootings in Tucson, which left six dead and 13 wounded.

"If we can continue to build on this symbolic gesture by emphasizing unity over division, I know that can have a real effect on the way that we work together," Udall told reporters Tuesday.

But like most ideas circulating in the fishbowl of Capitol Hill, the notion was quickly mocked, politicized, embraced, distorted and mocked again — all in about a week.

Republicans were initially skeptical of the idea, which threatened to deprive them of a televised visual of the new balance of power: a chamber filled with nearly 100 Republicans who weren't there last year.

Some suggested that the idea implied lawmakers were acknowledging partisan politics played some role in the shooting, a notion the Republicans strongly dispute.

But it soon became clear to lawmakers that the call for mixed seating was a symbolic opportunity neither party should pass up. (CNN actually polled the "issue": 72% of Americans said they wanted Democrats and Republicans to sit together.) And so began the scramble to get a date.

The Colorado delegation sat together. Democratic Sen. Charles E. Schumer of New York and Republican Sen. Tom Coburn of Oklahoma paired up. Udall teased the press with a "mystery seatmate," which turned out to be staunch conservative Sen. Jim DeMint (R-S.C.). The pair sat on the far right.

Meanwhile, Cantor asked Pelosi if she wanted to join him on the floor, but only after Pelosi had already invited Rep. Roscoe Bartlett (R-Md.). So Pelosi turned Cantor down, and he invited Rep. Robert C. Scott (D-Va.). They all tweeted about it Tuesday.

(What they did not tweet was that Cantor's invitation came hours after he had publicly criticized Pelosi for not being bipartisan enough — a move that probably didn't sweeten his invitation.)

In the hours before the speech, the hype over the new seating arrangement had escalated to a point that advocates held a news conference to talk about it. Sen. Lisa Murkowski (R-Alaska) called the whole thing "a little bit eighth grade" and worried that those covering the president's comments would "spend the whole evening focusing on who's sitting next to who."